We're currently adding on to the house and there will be 4 CAT6 outlets there and the current router we have (D-Link wireless) only has 4 ports on it. Basically, I just want to add more ports to it, but the only 8 port routers I saw are not wireless as well. I have a Linksys NH105 (link?) but I'm not sure if that would work - does a 5-Port Hub just split the incoming connection like I probably need? It's 10BaseT Ethernet, so it's just 10mbps which doesn't really matter with the basic cable connection we have to the internet (the CAT6 is just for the future now basically).

I also have an SMC7004ABR Barricade Router (link) but AFAIK, you can't plug a router into a router...

So what do I need? Do they have a D-LINK 8 port 802.11G router? I didn't see anything at D-Link's site (we have 2 computers running on D-Link wireless networking cards - so sticking with D-Link would be optimal).

1 Use your 4 port wifi router and uplink it to a switch for more wired ports
2 Buy an 8 port router and connect it to a WAP
3 Use your 4 port wifi router and at any local point use a bridge to switch for more connections

You can use either the hub or the SMC router to add more ports to your setup. Personally, I would not use a hub (especially only a 10Mbps one), as a switch is much more efficient and is cheap.

You can certainly connect a router to a router. In your case, you will only want to use the switch component of the SMC router. To do this, you will need to disable the DHCP server on the SMC router. You will also want to disable NAT routing on the SMC. The easiest way to ensure this is to connect your D-link router to the SMC via a LAN port on the SMC, rather than the WAN port. This will take NAT routing out of play. Of course, this will leave you with only 6 useable ports across the D-Link and SMC. If the SMC allows you to disable NAT routing via its web interface, you may be able to gain a port by using its WAN port instead, but that is not assured and may be a problem since many older routers only had 10Mbps WAN ports anyway.

But if I were to get a switch (four ports or six maybe) that would be easier? Just plug the switch into the D-Link router?

At extra cost, yeah a plain switch would be a little easier. However, if you are comfortable with configuring the SMC and if 6 ports will be enough, it is not really a problem to disable the DHCP server and connect to its LAN port. This would save you some $$$.

I use the 8 port Linksys switch plugged into my 4 port linksys router/switch and I have a Linksys WAP point plugged into one of the ports. I got the 8 port for $25 dollars after a staples rebate. Technically your "4 port router" is really a router with a 4 port switch. So the answer is buy a cheap switch and plug it in, keeping in mind it will use up one port on each end to connect the two together so 4 + 8 = 10 available ports or 4 + 4 = 6 available ports. In your situation the additional 4 port switch will be just right but you may want to get the 8 just to have future expansion capability.